The present invention generally relates to inspection of thermal batteries, more particularly, to systems, methods, and devices for determining whether a stack of pellets in a thermal battery are in a desired order.
Thermal batteries provide a large amount of energy relative to their volume. These batteries, if hermetically sealed, can be stored indefinitely without degradation of performance and can perform without preliminary preparation in many different environments. Thus, thermal batteries are a desirable source of power in a number of different applications. For example, thermal batteries are used in missile systems such as, JDAM, Stinger, Javelin, as well as other systems such as Aircraft Ejector Seats, and Sonar Buoys.
A typical thermal battery comprises one or more sections, with each section comprising one or more cells, and wherein each cell comprises a plurality of pellets. The pellets may, for example, be thin wafers of compacted powders forming the various components of the thermal battery. For example, the pellets may comprise an electrode, an anode, an electrolyte, a cathode, a pyrotechnic heat source, and/or the like. Thus, thermal batteries contain materials that are generally inert and non-conductive until the battery is activated.
Upon activation, the material becomes molten and highly conductive. This allows the cathode to interact with the anode. The thermal battery materials are activated by igniting the battery. For example, a mixture of iron powder and potassium perchlorate (the pyrotechnic heat source pellet) may be used to ignite a battery. Once activated, the battery may continue to perform until the active material is exhausted or until the battery cools below the melting point of the electrolyte.
Proper assembly of the thermal battery pellet stack insures proper function, specifically proper power output when the thermal battery is activated. Understandably, it is desirable to test thermal batteries in a manner that does not ignite the battery. One nondestructive thermal battery inspection test includes a test to verify that the components of the thermal battery are hermetically sealed. Other tests exist to verify the soundness of electrical connections within the thermal battery. For example, methods have been developed to test the soundness of electrical connections such as by use of insulation resistance, impedance, or capacitance tests. In another example, X-ray techniques have been used to create visual images of welds and to determine if a welded electrical connection was formed properly.
It is also important to test that the pellets are stacked in the correct order. In the past, efforts to test whether or not the pellets are stacked in the correct order have been visual in nature. In one such test, an operator visually confirms that the pellets are stacked in the correct order. This approach has associated drawbacks because the pellets are very thin and similar in appearance. Thus, from the side of the stack of pellets, it can be difficult to distinguish the pellets visually and determine if they are stacked in the proper order.
Some efforts have been made to automate this visual inspection through use of high resolution cameras and optical recognition techniques. Nevertheless, the same difficulties have been experienced in these automated visual inspection methods as in the human visual inspection methods. Thus, improved methods are needed for verifying that multiple components in a manufactured item were assembled in the proper order. In particular, improved methods are desired for verifying proper ordering in the pellet stack of a thermal battery.